Remove XMLTreeViewer virtual destructor
Remove XMLTreeViewer virtual destructor

Remove destructor for XMLTreeViewer as it was empty and marked as virtual.
This avoids creating an unnecessary vtable as this class is never subclassed
nor does it have any virtual function.

BUG=311561

Review URL: https://codereview.chromium.org/44533002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@160598 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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